Subcellular Architecture of Regulatory Protein Complexes at the Bacterial Pole Recent advances in microscopic imaging with single fluorescent molecules have led to super-resolution information providing the ability to observe objects with resolution beyond the standard optical diffraction limit of ~250 nm in the visible. At the same time, the complexity of bacterial organization has become more and more apparent, and given that the human body contains more prokaryotic cells than eukaryotic cells, it is essential to understand our microbial partners, for scientific benefit and for prevention of pathology. Much of the organization in ?-proteobacteria occurs in the cell pole, the anchor not only for the flagellum, but also for the chromosomal origin, the chemotactic apparatus and for critical regulatory and signaling subsystems that coordinate cell cycle progression. While approximate information is available about the cell pole, many mysteries remain, and high resolution information on the identity and precise relative locations of polar proteins is required to understand and ultimately influence bacterial biology. This application proposes a new line of research to understand the subcellular organization of regulatory proteins at the Caulobacter cell pole at unprecedented resolution. Such an effort requires the close integration of biochemical genetics with advanced three-dimensional (3D) super-resolution fluorescence imaging beyond the optical diffraction limit, in order to fully quantify the locations and spatial interactions of key proteins at the bacterial cell pole down to a precision of ~20-30 nm in x, y, and z. Caulobacter crescentus is a powerful model of cellular differentiation by virtue of its asymmetric cell division cycle, of which one of the PIs is expert. The new imaging methodology in which the other PI is expert relies on two components: (a) a two- color method for 3D imaging in cells with the double-helix point spread function (DH-PSF) microscope, which allows precise 3D imaging over a large depth of field, and (b) single-molecule active control microscopy, which provides super-resolution detail by sequentially imaging and localizing sparse subsets of individual emitters. Three thrusts define this program: Aim 1: Development of advanced two-color, 3D imaging with the DH- PSF microscope: Methods for localizing relative locations of pairs of polar proteins with precision extending down to ~20nm in x, y, and z will be developed and validated. Aim 2: Super-resolution 3D imaging of benchmark protein assemblies to define the coordinate system of the pole. The polar reference coordinate system will be defined by performing precise 3D imaging of TipN, McpA, CreS, and PopZ, key polar markers. Aim 3: Define 3D structural organization and dynamics of key regulatory protein assemblies at the bacterial cell pole. By combining an array of mutant strains with two-color 3D super-resolution imaging, we will establish the spatial organization of multiple pairs of regulatory proteins at the bacterial cell pole. Dynamical information in live cells will be extracted from imaging at differen times of the cell cycle, thus providing an unprecedented view of the structure as well as the dynamics controlling bacterial cell organization and function.